Processing using an ion beam is performed in manufacturing of various electronic components and the like. In particular, an ion implantation apparatus for implanting an impurity into semiconductor, an ion beam etching apparatus (hereinafter abbreviated as an IBE apparatus) configured to process a substrate by irradiating the substrate with an ion beam, and the like are widely used. Such an ion beam processing apparatus includes: an ion generation chamber being configured to generate plasma and thus serving as an ion source; a processing chamber in which a substrate is mounted on a holder and is subjected to processing; and an electrode assembly provided between the ion generation chamber and the processing chamber and configured to extract ions. The electrode assembly includes three electrodes each provided with numerous ion passage holes, and is known to use Mo (molybdenum), which is excellent in heat resistance and sputtering resistance, as the material thereof (see Patent Document 1).